1. Field of the Invention
The present invention relates generally to demodulators for frequency modulated (FM) signals and, more particularly is directed to a frequency demodulating circuit for a video reproducing apparatus, such as, a video tape recorder (VTR).
2. DESCRIPTION OF THE PRIOR ART
In a video tape recorder according to the prior art, a luminance signal is frequency-modulated, a chrominance signal is low-frequency-converted, that is, frequency converted to a frequency band below that of the FM luminance signal, and a mixed signal composed of the FM luminance signal and the low-frequency-converted chrominance signal is supplied to rotary heads and recorded thereby in slant record tracks on a magnetic tape. In such previously proposed VTR, in a variable speed playback mode thereof, such as, a still playback mode, a slow playback mode, a high speed playback mode or the like, the reproduced signal output from each rotary head is attenuated at each portion of the recorded signal where the magnetic head or heads cannot accurate1y scan the respective record track or tracks, with the result that a noise band is produced on the displayed picture. In other words, when the reproduced FM luminance signal is demodulated, a noise component, for example, due to head impedance noise and/or amplifier noise, is FM-modulated. Further, such noise is of a broad band so that the demodulated amplitude thereof becomes relatively large.
In order to reduce the noise on the picture, the frequency band of the FM luminance signal being supplied to the FM demodulator may be limited to a predetermined band near the carrier frequency. If such band-limiting of the input FM luminance signal is employed, the demodulated signal will become insufficient in those portions where the level of the reproduced signal output by the head or heads is substantially low, so that a reversing phenomenon will occur.
In order to prevent such reversing phenomenon in the reproducing or playback mode of a VTR, it has been proposed to supply the reproduced FM luminance signal from the rotary head or heads through an amplitude limiter to a high-pass filter which, when the level of the reproduced FM luminance signal is lower than that limited by the amplitude limiter, is effective to enhance the high band component. Such enhanced high band component is intended to avoid a black reversing phenomenon by which the level of the high band component is lowered and a so-called zero cross is not performed, as if the frequency of the reproduced FM luminance signal was considerably lowered. Further, in accordance with the prior art, the FM luminance signal derived from the high-pass filter is further amplitude-limited by an additional limiter prior to being applied to an FM demodulator for providing the respective luminance signal. In this connection, it is to be noted that the extent to which the black reversing phenomenon can be avoided depends on the order of the mentioned high-pass filter. However, if the order of such high-pass filter is increased for ensuring effective avoidance of the black reversing phenomenon, a white reversing phenomenon is caused by the additional limiter when a FM luminance signal having a relatively low frequency is input to the high-pass filter.